What is the difference between the audio gain and the green volume line (Camtasia 9)?

Camtasia 9 allows adjusting the volume of an individual clip with the green line. It is not clear what the difference is. I can increase the volume of a clip to a certain level with the green line and then add even more volume with the gain control. I can adjust multiple tracks with the gain, but this does not move the green line. But are these really doing the same thing or do they do something different (hidden) to the audio?